Phased cooling of the Siang antiform, Eastern Himalaya: Insight from multi-thermochronology and thermal studies,Journal of Earth System Science

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Phased cooling of the Siang antiform, Eastern Himalaya: Insight from multi-thermochronology and thermal studies
Journal of Earth System Science ( IF 1.9 ) Pub Date : 2021-02-18 , DOI: 10.1007/s12040-020-01541-7
Dnyanada Salvi , George Mathew , Kanchan Pande , Barry P Kohn

Abstract

The Siang antiform which forms the southern portion of the Eastern Himalayan Syntaxis is a massive subaerial duplex comprised of Paleogene rocks. The specifics of growth and deformation of the Siang duplex remain ambiguous due to limited studies in the region. Using multi-thermochronometry and Raman spectroscopy of carbonaceous matter (RSCM), this study place temporal depth constraints on the formation of the duplex. Results show that the cooling history of the northern part of the Siang antiform is separate from the central and proximal zone. The study utilises 09 new biotite ⁴⁰Ar/³⁹Ar and 05 zircon (U–Th)/He (ZHe) ages. The new data is complemented with our earlier published (Salvi et al. in Geomorphology 284:238–249, 2017) 09 ZHe and 11 AFT ages and 02 ZHe ages of Liebke et al. (Geol. Soc. London, Spec. Publ. 353:71–97, 2011). Biotite ⁴⁰Ar/³⁹Ar cooling ages suggest that the distal end of the MCT zone in the Siang window was active at least till ca. 11 Ma. The ZHe cooling ages ca. 10–8 Ma intimate exhumation due to Lesser Himalayan duplexing on a shallower (~7–8 km) MHT. The creation of several duplexed antiforms by the Paleogene rocks on the emplaced MBT thrust sheet led to doming up of the roof sheet. Erosion through the roof sheets exposed the Paleogene rocks presently seen in the Siang window. The peak metamorphic temperatures decrease from 650–400°C in the lower LHS to 250–300°C in the upper LHS, and < 200°C in the sub-Himalaya. The RSCM results corroborate thermochronological ages and inform that the northern part of the metamorphosed lower LHS rocks exhumed from greater depths, >20 km during early-middle Miocene. While in the central and southern regions, the rocks exhumed from comparatively shallower depths of ~7–8 km since the late Miocene. We suggest ~3–4 km of the cover rocks have been removed since Pleistocene.

Highlights

1.
First multi-thermochrometry studies suggest distal end of MCT zone in the Siang window was active till ca. 11 Ma.
2.
ZHe cooling ages ca. 10–8 Ma intimate exhumation due to Lesser Himalayan Duplexing on a shallower (~ 7–8 km) MHT.
3.
The creation of several duplexed antiforms by the Paleogene rocks on the emplaced MBT thrust sheet led to doming up of the roof sheet.
4.
RSCM peak metamorphic temperatures furnish 650–400 °C in the lower LHS to 250–300 °C in the upper LHS, and < 200 °C in the sub-Himalaya.
5.
Average exhumation rate post Late Pliocene is ~ 1.6–3.3 mm/a in the Siang window, increases to 4.0–6.6 mm/a in the last 1.0 Ma, north of Tuting.

中文翻译：

喜马拉雅东部相安反型的分阶段冷却：多重热年代学和热学研究的启示

摘要

形成东喜马拉雅山脉南部的Siang反型是由古近系岩石组成的大型地下双相体。由于对该地区的研究有限，Siang复式植物的生长和变形的具体细节仍然不明确。使用多时相计时和碳质物质的拉曼光谱（RSCM），本研究对双链体的形成施加了时间深度约束。结果表明，Siang反型地貌北部的冷却历史与中部和近端区分开。该研究利用了09年新的黑云母⁴⁰ Ar / ³⁹ Ar和05锆石（U–Th）/ He（ZHe）年龄。新数据得到了我们之前发表的文献的补充（Salvi等人，《地貌学》，284：238–249，2017）Liebke等人的09 ZHe和11 AFT年龄和02 ZHe年龄。（伦敦，伦敦，规范出版社，353：71-97，2011年）。黑云母⁴⁰ Ar / ³⁹Ar冷却年龄表明，Siang窗口中的MCT区的远端至少在大约20年前是活跃的。11 Ma。ZHe冷却时间约为由于在较浅（〜7–8 km）的MHT上进行了较小的喜马拉雅双工，因此发生了10-8 Ma的亲密掘尸。由古近纪岩石在已放置的MBT推力板上形成的几种双工反型导致顶板隆起。屋顶上的侵蚀使目前在Siang窗户中看到的古近纪岩石暴露出来。峰值变质温度从较低的LHS的650-400°C降低到较高的LHS的250-300°C，而在喜马拉雅山以下则小于200°C。RSCM的结果证实了热年代学年龄，并告知变质的低层LHS岩石的北部在中新世中期至中部> 20 km时从更大的深度中挖出。在中部和南部地区自中新世以来，岩石从大约7-8 km的较浅深度中挖掘出来。我们建议，自更新世以来约3-4 km的覆盖岩已被清除。